Polylactide and apatite compositions and methods of making the same

ABSTRACT

A method is provided for synthesizing PLA/apatite composites with improved mechanical strength. In one aspect, a calcium-phosphate/phosphonate hybrid shell is developed to incorporate more reactive hydroxyl groups onto hydroxyapatite (HA) particles. PLA is covalently bonded to HA calcium phosphate hybrid shell, creating a strong interphase between HA and PLA, thus significantly improve the mechanical strength in comparison to that of non-modified HA.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. national phase application filed under 35U.S.C. § 371 of International Application PCT/US2013/029839, filed Mar.8, 2013, designating the United States, which claims the benefit of U.S.Provisional Application Ser. No. 61/623,483, filed Apr. 12, 2012, whichare incorporated herein as if fully rewritten, and also claims thebenefit of U.S. Provisional Application Ser. No. 61/623,490, filed Apr.12, 2012, which is incorporated as if fully rewritten herein.

FIELD

The present application is directed to compositions includingpolylactides and calcium phosphate composites and methods for making thesame. More particularly, the synthesis methods and resulting compositesinclude polylactides and apatite that have been modified when combined.

BACKGROUND

Bioresorable compositions such as polylactides (PLA) are useful for bonefixation and bone repair and have the advantage of not requiringsurgical removal after the bone heals. However, the use of polylactidesfor bone fixation and bone repair can lead to a variety of undesirableside effects, such as inflammation or allergic reactions.

Combining apatite, such as hydroxyapatite (HA), with PLA yields acomposition similar to the composition found in bone and teeth in vivo.Polylactide/hydroxyapatite (PLA/HA) composites facilitate theosteoconductive properties of an implant plus aides in lessening theside-effect of the PLA composite by neutralizing its acidic bio-degradedby-products. PLA/HA composites have the potential of improving clinicalbone healing, but current PLA/HA composites have a significantdisadvantage due to their mechanical weakness. This weakness eliminatesthe use of PLA/HA composites in load-bearing areas. It is generallysuspected that PLA/HA composite weakness is caused by the weakinterphase between PLA (hydrophobic) and HA (hydrophilic) structures.

Current PLA/HA composites are prepared by several different methods suchas direct blending using nonmodified HA, solution co-precipitation,emulsion, and mechanical mixing. Because of the relatively highhydrophobicity of PLA and hydrophilicity of HA, obvious problems ofthese methods include weak interfacial adhesion between HA and the PLAmatrix and agglomeration of the HA particles in the matrix. Lack ofadhesion between the two phases will result in an early failure at theinterface between PLA and HA, usually leading to weak mechanicalproperties. As an example, the tensile strength of PLA/HA compositesdecreased significantly from 54 MPa for pure PLA to 41 MPa even with aHA content of only 18%.

Increasing interfacial bond strength between PLA and HA is an importantfactor on a matrix interface to achieve increased mechanical strength.In the last decade, coupling agents such as silanes, isocyanates, andorganotitanates have been used to improve the interfacial adhesionbetween certain ceramic fillers and different polymeric matrices.Although the effects on alumina and silica systems (SiO₂, bioglass,clay, etc.) were encouraging, the feasibility of using these agents togain improved interfacial adhesion to HA was not confirmed.

SUMMARY

It has been unexpectedly found that a polylactide having a generalformula of —(OCH(R)CO)_(n)—, wherein R=H or C1-C10 alkyl and whereinn=1-4, may be combined with an apatite material having a general formulaof Ca₁₀(PO₄)₆X₂ where X is OH or F or both, whereby the apatite ismodified prior to coupling with the polylactide. In one form, thematerial may be prepared by combining a hydroxyapatite source having ageneral formula of Ca₁₀(PO₄)₆(OH)₂ with an organic material withphosphonic acid functionality having a general formula of —PO(OH)₂ toform an intermediate hydroxyapatite phosphonic acid containing material.The intermediate hydroxyapatite phosphonic acid containing material,which is a reaction product of the hydroxyapatite source and the organicmaterial with phosphonic functionality may then be combined with alactide material having a general formula of (OCH(R)CO)_(n), whereinn=1-4, preferably 2=n, or CH(OH)(R)COOH, wherein R=H or C1-C10 alkyl, toform the polylactide/hydroxyapatite material.

Further, the polylactide/modified phosphonic acid apatite material whichhas been reacted with polylactide may then be combined with additionalpolylactide to form a composite polylactide/apatite material. It isbelieved that such a composite polylactide/apatite material may haveincreased tensile strength compared to a polylactide/apatite materialthat has not been modified with a phosphonic acid containing material.The amount of additional polylactide to polylactide/modified phosphonicacid apatite material in the composite may range from about 1% weight toabout 99% weight. In one form, the amount of polylactide/modifiedphosphonic acid apatite material combined with additional PLA issufficient to affect an increase in a tensile strength of the compositeof at least about 50% when compared to a composite which does notinclude modified phosphoric acid apatite material as described herein.According to another form, the amount of polylactide/modified phosphonicacid apatite material is sufficient to effect a 100% increase in tensilestrength when compared to a composite which does not include modifiedphosphonic acid apatite material.

According to one form, the apatite source is modified with thephosphonic acid containing material to introduce —OH and/or NH₂ groupson the surface of the apatite source to form the apatite material whichis reactive with polylactide. In one form, the intermediate apatitephosphoric acid containing material undergoes surface initiatedpolymerization with the lactide groups of the polylactide via —OH and/orNH₂ groups found on a surface of the intermediate apatite phosphoricacid containing material.

The method may also include the step of separating unreacted phosphonicacid containing material from the intermediate apatite material.Similarly, the method may include the step of separating unreactedlactide containing material from the polylactide/apatite material.

In one form, the phosphonic acid containing organic material includesN-(2-hydroxyethyl) iminobis(methylphosphonic) acid (HIMPA). In thisregard, in one form, the hydroxyapatite source is suspended in anaqueous solution of N-(2-hydroxyethyl) iminobis(methylphosphonic) acid.In another form, the hydroxyapatite is precipitated in the presence ofN-(2-hydroxyethyl) iminobis(methylphosphonic) acid. However, other formsof phosphorous containing compounds also may be used.

It has been found that there are significant benefits of this method ofsynthesis, such as significantly increased tensile strength whencompared to that of polylactide alone.

By one approach, the composition comprises significantly higherpolylactide covalently attached to hydroxyapatite than that of currentcompositions produced by conventional methods.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates X-ray diffraction patterns of crystallized HAcomposites prepared according to different methods;

FIGS. 2A-2D illustrate thermogravimetric analysis (TGA) curves of HAcomposites prepared according to different methods;

FIG. 3 illustrates pH dependent ζ-potential and particles size profilesof HA composites prepared according to different methods;

FIG. 4 illustrates Diametral Tensile Strength (DTS) of PLA/HA compositesprepared according to different methods; and

FIG. 5 illustrates a schematic representation of PLA/HA compositepreparation using initiated polymerization.

DETAILED DESCRIPTION

Described herein are synthesis methods and compositions comprisingPLA/HA composites. The general method provides synthesis of PLA/HAcomposites by grafting PLA on an HA intermediate, such as viasurface-initiated polymerization (SIP) through the non-ionic surfacehydroxyl groups. It should be noted that when referring to a compositematerial, the material includes modified phosphonic acid apatitematerial along with additional apatite material.

In one form, the synthesis methods described herein use a method forpreparing a polylactide/apatite material comprising the steps ofcombining an apatite source with a phosphonic acid containing materialto form an intermediate apatite material. In one form, the intermediateapatite material is formed thereby introducing a plurality of —OH and/orNH₂ groups coupled to the apatite. The intermediate apatite material maythen be combined with a lactide containing material to form thepolylactide/apatite material which has has a diametral tensile strengththat is at least 1.5 times the diametral tensile strength of apolylactide/apatite material prepared without combining a apatitematerial with the organic material having phosphonic acid functionality.

The apatite material may include a variety of apatite containing and/orproviding materials prepared in a variety of manners. In one form, theapatite material has a general formula of Ca₁₀ (PO₄)₆X₂ where X is OH orF or both OH or F. In this regard, the apatite material may befluoroapatite and/or hydroxyapatite. According to one form, thehydroxyapatite material has a general formula of Ca₁₀(PO₄)₆(OH)₂. In oneform, the hydroxyapatite material includes purified HA which may beprepared as described below.

Other forms of calcium phosphate containing materials may also be usedin lieu of HA or in combination with HA. For example, such materialsinclude, but are not limited to monocalcium phosphate (MCP, Ca(H₂PO₄)₂),dicalcium phosphate (DCP, CaHPO₄), tricalcium phosphate (TCP,Ca₃(PO₄)₂), amorphous calcium phosphate (ACP, Ca₃(PO₄)₂), tetracalciumphosphate (TTCP), fluoroapatite (FAP, Ca₁₀(PO₄)₆(OH,F)₂), octacalciumphosphate (OCP, Ca₈H₂(PO₄)₆).

The apatite and/or calcium phosphate containing material may be used ina variety of amounts. For example, the modified apatite and/or calciumphosphate material may be used in an amount of about 1% weight to about99% weight based upon the weight of the reaction product of modifiedapatite and/or calcium phosphate material and lactide. In another form,less than about 60% is used in the overall composite material. It shouldbe noted that a number of the descriptions and examples below describethe use of HA, but other apatite materials and calcium phosphatematerials may similarly be used.

The organic material with phosphonic acid functionality may include avariety of different materials, such as phosphorous containingmolecules, including N-(2-hydroxyethyl) iminobis(methylphosphonic) acid,hydroxyethylhophonic acid, any other hydroxyl or amino containingphosphonic acid, phosphoric acid or any kind of phosphorous containingcompound. According to one form, the organic material with phosphonicacid functionality has a general formula of —PO₃H₂. In one form, thesource of phosphonic acid includes N-(2-hydroxyethyl)iminobis(methylphosphonic) acid. The amount of phosphonic material willbe determined by the amount of phosphate to precipate the apatite. Themolar ratio of phosphonic to phosphate may be ⅕ or lower. The phosphonicagent may contain either —OH or —NH2 or both, which can initiatepolymerization of the polymers.

The lactide used in the method may also include a variety of differentsources. For example, the lactide may include a material has a generalformula of —(OCH(CH₃)CO)_(n)—, such as PLA (polylactide). Other polymerscan be used to composite with HA include polycaprolactone (PCL),polyglycolide (PGA), PLGA (polylactide-co-glycolide),polyhydroxybutyrate or poly(hydroxyvalerate or poly(carbonates) orpolyphosphazene or polyanhydrides or other polyesters or polyurethaneand natural origin degradable polymers such as cellulose or starch orgelatin or chitosan or peptides and their derivatives. The lactidematerial may be used in a variety of amounts, such as up to about 99weight % of the overall composite material as described in FIG. 5C. Inone form, about weight 50% lactide material is used.

The method may also further comprise the step of separating unreactedphosphonic acid containing material from the intermediate hydroxyapatitematerial by washing with an aqueous solution, such as regular water,preferably distilled water.

The polylactide/apatite composite material can further comprise the stepof combining the polylactide/apatite material with additionalpolylactide to form a composite polylactide/apatite material. Thepolylactide/apatite can be separated by precipitation in a solvent suchas chloromethane, choroform, preferably methylene chloride, andprecipitated with an excess of methanol, propanol, acetone, preferablyethanol, and then dried in vacuum to remove the residual solvent.

In one form, a calcium-phosphate/phosphonate hybrid shell intermediateis formed thereby creating a greater amount of reactive hydroxyl groupsonto the HA moieties. This structure may be formed through a phosphonicbased bi-dentate chelating agent that bonds to the HA surfaces.Afterwards, PLA is covalently grafted from the HA through the increased—OH groups by a phosphonic agent. Improved mechanical properties of thePLA/HA composite results from the chemical bonding of the phosphonicgroup increasing the activity of surface —OH groups on the HA, at thesame time surface-initiated polymerization between the HA and PLAparticles improves the HA/PLA interface.

Preparation of HA

In one form, the HA was synthesized by solution reaction of Ca(OH)₂ andH₃PO₄ (or CaCl₂ or Ca(NO₃)₂ or any other calcium containing salt andNa₃PO₄ or any other phosphate containing salts). In brief, about300-1000 mL, preferably 500 mL of distilled water was boiled in aTeflon-coated pot, equipped with an electric stirring paddle and areflux condenser with a CO₂-absorbing NaOH trap to protect fromatmospheric CO₂, under Ar or nitrogen gas for range 10-60 min. One moleof CaO was added to the water, and 300 mL of H₃PO₄ solution (2 mol/L)was slowly (about 0.5 mL/min) added to the Ca(OH)₂ slurry to obtain afinal Ca/P molar ratio of about 1.67. The reacting mixture was boiledfor two days. The precipitated solid was collected by centrifugation andwashed with distilled water. The solid was re-dispersed in boileddistilled water and was re-boiled for another two days. These washingand boiling procedures were repeated until the pH of the supernatant wasabout 6. At pH 6, any traces of anhydrous dicalcium phosphate (DCPA)that might have formed due to possible local more acidic environmentswere converted to HA. In some cases, the HA precipitate, collected bycentrifugation, was used for phosphonic acid coating. In other cases,the HA precipitate was collected by centrifugation, washed with anorganic solvent, e.g. methanol, ethanol, preferably acetone, and driedat about 110° C.

The HA may be modified in a number of different manners to form the HAintermediate. For example, the HA may be modified by coating thematerial and/or the HA may be modified by co-precipitation.

In an aspect for preparing the PLA/HA composite, the HA intermediate isprepared by coating with —OH groups using surface modification(PLA-HA-HIMPA-A) (method A which produces a monolayer surfacemodification as shown in FIG. 5). An exemplary method suspends HAparticles in an aqueous solution of N-(2-hydroxyethyl)iminobis(methylphosphonic) acid (HIMPA) (about 2.5%) at a HA/HIMPA massratio of about 5:1 or higher. HIMPA was then used as a bidentatechelating agent to link the non-ionic hydroxyl groups to HA (method A).

In another form, HA intermediates are prepared by coating —OH groupsusing in situ co-precipitation (PLA-HA-HIMPA-B). In this method (methodB which produces core shell structure as shown in FIG. 5), HA wasprecipitated in the presence of HIMPA. In an exemplary method, thephosphate groups on the HA surface were partially substituted byphosphonic acid groups, the Ca/(PO₄ ³⁻+PO₃H₂) molar ratio is 1.67 forHA. It should be noted that in one form, the molar ratio of calcium tophosphorous may be 5/3. Additionally, in one form, the molar ratio ofPO₄ ³⁻/PO₃H₂ may be 5/1 or higher.

In one form, PLA was grafted on the above prepared HA intermediateparticles using surface initiated polymerization (SIP) (Hong et al.,“Nano-composite of poly(L-lactide) and surface grafted hydroxyapatide:Mechanical properties and biocompatibility,” 2005, Biomaterials26:6296-6304). An exemplary grafting method involved suspending HA in 20ml of toluene containing 10 μL of SnOct₂ acting as a catalyst; andseparately dissolving 2 g of L-lactide in 10 mL of dry toluene ordimethylformamide (DMF). The HA suspension was heated to about 90° C.,and then dropped into the L-lactide solution under argon protection andwith stirring. Argon protects against ring-opening polymerization oflactide, which is sensitive to moisture and impurities. After thereaction continued at about 140° C. for about 48 hours, the reactionmixture was cooled down to room temperature. The PLA-grafted-HA(PLA-g-HA) particles were separated by centrifugation and washed withexcess volumes of methylene chloride to remove the free PLA that did notgraft on the surface of the HA particles.

According to one form, PLA/PLA-g-HA composites were synthesized fromPLA-g-HA and additional PLA. An exemplary method involved dispersingnon-treated HA or PLA-g-HA with PLA in a ratio of ¼, with a varied rangefrom 0% to 100% dispersed in methylene chloride and mixed vigorously forabout three hours. The composite was precipitated with an excess ofethanol to remove the residual solvent.

The synthesis methods may also include the step of separating unreactedphosphonic acid containing material from the intermediate hydroxyapatitematerial. Similarly, the method may include the step of separatingunreacted lactide containing material from thepolylactide/hydroxyapatite material.

EXAMPLES Example 1

HA coated with —OH groups prepared by using in situ co-precipitation wascharacterized by powder X-ray diffraction (XRD). XRD was used todetermine the crystalline phases and crystallinity of these phasespresent in the HA/PLA composites. As found in FIG. 1, pure HA displays atypical crystalline HA prepared by a current precipitation method. Thepattern of the HIMPA coated HA prepared by the above described methodusing surface modification (method A) shows a slight decrease incrystallinity when compared to that of pure HA. In comparison, theHA-HIMPA prepared by using in situ co-precipitation (method B) exhibitedmore discernable peak broadening, signifying significantly lowercrystallinity, as a result of partial substitution of the phosphate inHA by the phosphonate.

Example 2

Thermogravimetric analysis (TGA) was used to estimate the amount ofHIMPA coated on HA intermediates. FIG. 2A shows the TGA curves of pureHA and HIMPA coated HA. Both the pure HA and HIMPA coated HA wasprepared by the above described surface modification (method A) showed asimilar 2.5% mass loss when heated to 600° C., showing that the amountof the HIMPA coating on HA was too low to be observed by TGA. Incontrast, the HA-HIMPA prepared by using in situ co-precipitation(method B) showed a 5.5% mass loss (see FIG. 2A), illustrating asignificant amount of HIMPA coating. Based on the mass loss differencesamong the pure HA and the HA-HIMPA prepared by surface modification andin situ co-precipitation, the HIMPA content on HA-HIMPA prepared by insitu co-precipitation was estimated to be about 3%. These resultssupport the concept that phosphonic acid can be more effectively coatedon HA using in situ co-precipitation method (method B), by formation ofa hybrid Ca/(PO₄ ³⁻+R—PO₃ ²⁻) intermediate shell over the core of HA.

Example 3

The amount of PLA grafted on HA was also evaluated by TGA. FIG. 2A showsthe TGA curves for samples of PLA grafted onto HIMPA coated HA samplesby method A (PLA-HA-HIMPA-A), using either DMF or toluene as thesolvent. The mean values of mass loss for both PLA grafted samples wereabout 4% mass fraction, which are greater than the 2.5% mass loss forthe pure HA and HA-HIMPA-A samples. This demonstrated that a smallamount of PLA (1.5% mass fraction) can be grafted on HA either in DMF ortoluene, with toluene being a more effective solvent. FIG. 2B shows theTGA curves for the sample series using the in situ co-precipitationmethod B. The mean values of mass loss at 600° C. of HA, HA-HIMPA-B,PLA-HA-HIMPA-B-DMF and PLAHA-HIMPA-B-Toluene were 2.5%, 5.5%, 8.5% and12.5%, respectively. Thus, the amounts of PLA coating on the HAparticles prepared by method B in DMF and toluene were approximately 3%and 7%, respectively. These values are 2 and nearly 5 times,respectively, those produce by method A. The data showed that method Btogether with toluene as the solvent can efficiently produce a largeamount of PLA onto the HA particles. FIG. 2C shows the first derivativesof the TGA (DTG) curves of the same samples from FIG. 2B. The pure HAexhibited a relatively flat curve except for a broad peak around 320° C.

In contrast, HA-HIMPA-B shows a large peak around 450° C., due to theloss of HIMPA that was incorporated within the HA. PLA grafted HA fromboth solvents showed the same mass loss profile around 450° C. Inaddition, PLA HA-HIMPA-B-toluene shows a significant peak around 260°C., which can be attributed to the loss of PLA. The sharpness of thispeak provides evidence of a large amount of PLA on HA. The above resultsindicate that phosphonic acid (HIMPA) can be used as an efficientcoupling agent to coat HA particles, especially by using in situco-precipitation of HA in the presence of HIMPA (method B). PLA can alsobe grafted on HIMPA coated HA through surface initiated polymerization.Due to the greater amount and different kind of —OH groups on theHIMPA-HA than that of uncoated HA, more PLA can be grafted onHIMPA-coated HA by using method A or method B than on HA alone.Furthermore, the amount of grafted PLA by method B is greater than thatof method A, suggesting that HIMPA coating produced by method B is amore efficient approach to graft more PLA onto HA particles.

Example 4

The pH dependent ζ-potential and particles size profiles providedfurther information regarding grafting of PLA onto the HA. The weightedaverage value (n=3) of median particle size and ζ-potential of thenon-treated HA and PLA grafted HA (HA-PLA) are shown in FIGS. 3A and 3B,respectively. At pH 13, HA presented a negative ζ-potential (FIG. 3A).These negative net surface charges prevented the agglomeration of HA,resulting in an average particle size of approximately 4 μm. Titrationfrom pH 13 to pH 4 led to a gradual change of potential from −25.1 mV to−0.6 mV (FIG. 3A). Due to the decreasing ζ-potential, HA particles tendto conglomerate, resulting in an increase in the average size of HA from4 μm (pH 13) to 12 μm (pH 4). The sudden decrease in the size of HAbelow pH 4 is caused by the significant dissolution of HA in the highlyacidic solution. Unlike HA, the HA-PLA showed distinctly different pHdependent ζ-potential and particle size profiles (FIG. 3B). At pH 13,HA-PLA and HA present similar negative S-potential. However, upontitration of acid, ζ-potential of HA-PLA shows a positive peak (−5.43mV) at around pH 8, corresponding to the least net surface charge.Additionally, acid titration led to increases in negative charges andreached a peak at pH 5, which then decreased with further decreases inpH. In addition, the mean size of HA-PLA remained nearly constantbetween pH 11 and pH 5, this is consistent with the concept that PLAcoating significantly altered the surface charge and agglomerationproperties of HA. The association of —COO— with H+ at below pH 5 reducedthe net surface charge of HA-PLA, causing particle agglomeration andincreasing the particle size from 4 μm at pH 5 to 8 μm at pH 2. The PLAcoating protected the HA from rapid dissolution in strong acidicenvironments.

Example 5

FIG. 4 shows the load-strain curves and corresponding DTS values for thePLA/HA composites. PLA/HA composite from non-treated HA was (17.4±1.0)MPa (mean±standard deviation; n=3), which is significantly lower thanthat (30.3 MPa, unpublished data) of the PLA alone samples prepared fromthe same polymer. The decrease in strength, which is in agreement withthe literature results, can be attributed to the weak interfacialadhesion between the PLA matrix and the non-treated HA.

In contrast, the DTS of the two composites prepared from interfaciallyimproved HA (PLA/HA-PLA-A and PLA/HA-PLA-B) were (37.3±1.4) and(38.3±2.3) MPa, respectively. These values are more than twice (p<0.05)that of the composite from non-treated HA (17.4 MPa) and 23% higher thanthat of the PLA itself. The increased DTS values are also higher thanthe DTS values of composites with a similar HA ratio. These resultsdemonstrate that combination of SIP with the phosphonic acid agent cansignificantly improve the tensile strength of the PLA/HA composite.Because HIMPA can lead to a strong interfacial binding between HA andnon-ionic —OH groups of HIMPA, the PLA initially grafted on thecoated-HA can be considered as covalently bond to HA, and the mechanicalproperties of PLA/HA can be significantly improved.

Disclosed herein are methods of improving the interfacial interactionsbetween PLA and HA by grafting PLA directly from the surface of HA viasurface initiated polymerization (SIP). This approach consists ofseveral conceptual steps as depicted schematically in FIG. 5. Thelactide monomer was initiated by the non-ionic —OH groups on the surfaceof HA in the presence of Sn(Oct)₂ as the catalyst (FIG. 5b ). Becauseliterature data indicated that the innate hydroxyl groups (—OH) on thesurface of HA may not be sufficiently reactive and the amount of PLAthat can be grafted was limited, modification of the HA surface toincrease the amount and the type of —OH groups with greater reactivitywas necessary to improve PLA grafting.

In one form, HIMPA, a molecule with two phosphonic acid groups on oneend and an —OH group on the other end, was used as a bidentate chelatingagent to link the non-ionic hydroxyl groups to HA (FIG. 5a ). It issuspected that HIMPA may lead to a stronger binding with HA, and ahigher amount of HIMPA can be coated on HA than that of mono-phosphonicacid molecule. Of the two approaches used in the present study to attachnon-ionic —OH groups on HA particles, method B, an in situco-precipitation method, in which HA was precipitated in the presence ofHIMPA, led to a significantly higher HIMPA incorporation than that bymethod A. The resulted HIMPA coated HA from method B showed a lowercrystallinity than that from method A and standard HA (FIG. 1),suggesting that some phosphonate ions were embedded within the HAcrystal lattice, possibly forming a hybrid shell of Ca/(PO₄ ³⁻+R—PO₃²⁻).

Initiated by the surface non-ionic hydroxyl groups from the HIMPAcoating, PLA could be successfully grafted from HA by using surfaceinitiated polymerization in the presence of SnOct₂. Because ring-openingpolymerization of lactide (LA) is sensitive to moisture or impurities inthe solvent or reactants, in one form, all the solvents includingtoluene and DMF must be dried over sodium or calcium hydride,respectively. Due to the reactivity of the surface —OH from HIMPA coatedHA, the amount of grafted PLA either from method A or method B wasgreater than that of non-modified HA. In particular, the greater amountof HIMPA coating formed on HA by method B led to a higher amount ofgrafted PLA (7% mass fraction), which was higher than that previouslyreported in the literature (5%). The combination of phosphonic acidcoupling agent and surface initiated polymerization facilitated the PLAto link with the HIMPA coated HA surface through covalent bonding, and astrong interfacial adhesion can thus be established.

Due to the improved interface and the entanglement of the PLA on thesurface of HA and the PLA matrix, the mechanical properties of PLA/HAcomposites prepared from PLA-grafted HA was significantly improved incomparison to that of the composite prepared using non-grafted HA (17MPa). This suggests that the interfacial improvement plays a role in themechanical properties of the composite. The design of biocomposites ismore complex than that of conventional monolithic materials because ofthe large number of design variables that must be considered. It isbelieved that the mechanical properties of PLA/HA composite are affectedby the inherent characteristic of PLA such as chemical configuration,crystallinity, relative molecular mass and polydispersity index, andcharacteristics of the HA filler such as morphology (particulate orwhisker), size distribution, crystallinity (amorphous or crystalline),preparation method (sintered or solution precipitated). Additionalconsiderations include mass fraction of HA, composite preparationmethods (solvent casting, hot pressing, compression molding, meltextrusion, biomimetic process, etc.), interfacial treatment as well asspecimens fabrication techniques and conditions (heat pressing, casting,sintering, machining, together with molding temperature, pressure, andprocessing time).

In order to specifically demonstrate the effect of interfacialimprovement proposed in this application, the composites for this studywere prepared from the same PLA under the same experimental condition,e.g., filler ratio, composite technology, temperature, molding pressure,etc., with the interfacial optimization of the HA particles (non-treatedHA or PLA grafted HA) being the only difference.

The foregoing descriptions are not intended to represent the only formsof the compositions and methods according to the present application.The percentages provided herein are by weight unless stated otherwise.Changes in form and in proportion of components, as well as thesubstitution of equivalents, are contemplated as circumstances maysuggest or render expedient. Similarly, while compositions and methodshave been described herein in conjunction with specific embodiments,many alternatives, modifications, and variations will be apparent tothose skilled in the art in light of the foregoing description.

What is claimed is:
 1. A method for preparing a polylactide/apatitematerial comprising the steps of: combining an apatite material selectedfrom the group consisting of a phosphate having a general formula ofCa₁₀(PO₄)₆X₂, where X=—OH or F, monocalcium phosphate, dicalciumphosphate, tricalcium phosphate, amorphous calcium phosphate,tetracalcium phosphate, fluoroapatite, octacalcium phosphate with anorganic material having phosphonic acid functionality to form anintermediate apatite and phosphonic acid containing material; andcombining a lactide material having a general formula of —(OCHRCO)_(n)—where R=Cl alkyl or —(OCH₂CO)_(n)— where n=1-4, or copolymers thereof,and mixtures thereof with the intermediate apatite and phosphonic acidcontaining material and using surface initiated polymerization oflactide onto the intermediate apatite and phosphonic acid containingmaterial via —OH or —NH₂ groups on a surface of the intermediate apatiteand phosphonic acid containing material to form the polylactide/apatitematerial.
 2. A method for preparing a polylactide/apatite materialcomprising the steps of: combining an apatite material selected from thegroup consisting of a phosphate having a general formula ofCa₁₀(PO₄)₆X₂, where X=—OH or F, monocalcium phosphate, dicalciumphosphate, tricalcium phosphate, amorphous calcium phosphate,tetracalcium phosphate, fluoroapatite, octacalcium phosphate withN-(2-hydroxyethyl) iminobis(methylphosphonic) acid to form anintermediate apatite and phosphonic acid containing material; andcombining at least one component selected from the group consisting of amaterial having a general formula of —(OCHRCO)— where R=Cl alkyl or—(OCH₂CO)_(n)— where n=1-4, or copolymers thereof, and mixtures thereofwith the intermediate apatite and phosphonic acid containing material toform the polylactide/apatite material.